They are sketches by sailors and drawings by priests – lines on paper hundreds of years old that show islands and coastlines whose shapes often look decidedly different from their modern representation.

But across Asia, those centuries-old maps have become objects of modern desire as countries look to the past for ammunition in a battle over ownership rights in waters vital to trade and defence.

On Tuesday, a court in The Hague is expected to rule on a key fight over maritime ownership in the South China Sea between China and the Philippines. The decision by the little-known Permanent Court of Arbitration will form a major international test of the complex and conflicting claims that countries have laid over the region's waters, and in particular China's bid to own most of the South China Sea.

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But behind the legal arguments before the arbitration tribunal, Asian nations have been engaged in a high-stakes bid to gather symbolic proof of rightful ownership, which has great value in the court of public opinion. To do that, they have turned to old maps, seeking confirmation from the fading scribblings of mariners, functionaries and scholars – and, in at least one case, a stone inscription nearly a millennium old – of who rightfully owns what.

"There has been a noticeable uptick in interest in European maps that assign certain East Asian islands to one regional group or another," said Kevin Brown, the founder of Geographicus Rare Antique Maps in Brooklyn.

Chinese depictions, too, have become valuable commodities, since it is Beijing's unbending imposition of its claims through the construction of hotly disputed artificial islands that has brought long-standing international arguments to a boil. The priciest map in Mr. Brown's current inventory is the earliest known print of the All-Under-Heaven Complete Map of the Everlasting Unified Qing Empire, listed at $250,000 (U.S.).

Over the last decade, prices for rare Chinese maps have risen five and even 10 times in value, dealers say, mostly because wealthy Chinese have begun to collect.

But the parallel rise in demand for aged portrayals of maritime territory offers a window into the huge effort countries have put into buttressing their claims to islands and waterways that hold natural resources, shipping routes and, for many nations, national pride.

As a visual symbol, little matches a map's potency at showing dominion over land and sea. In 2003, the U.S. Library of Congress paid one of the highest prices ever for a map, spending $10-million to acquire the first-ever document to use the term "America."

For China, "it's about constructing a superpower. They want to show that China is big and powerful. And so every inch of territory you can claim as Chinese is awfully good," said Timothy Brook, a historian at the University of British Columbia who specializes in China.

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"They search for maps as though there's some kind of magic proof of a political claim," he said.

The most prominent is a relatively recent one, a 1947 map published by the Republic of China – the government that later fled to Taiwan from Mao's Communists – that placed 11 long dashes in a curving U-shape encompassing virtually the entire South China Sea, a 3.5-million-square-kilometre body of water.

Modern Taiwanese leadership has said the lines (subsequently reduced to nine, then raised to 10) are only meant to show which islands are Chinese. But in 2009, Beijing told the United Nations it also claims the entire surface and seabed contained inside its massive reach.

The race for old maps comes partly in response, as China's neighbours seek counter-evidence – "mostly because historic cartography does not support China's claims," Mr. Brown said.

In the Philippines, a senior Supreme Court justice, Antonio Carpio, conducted a lengthy review of 60 historical maps, some of them stone and silk drawings dating to 1136. Each of the Chinese documents he amassed depicts China extending only as far south as Hainan Island, whose shores lie more than 1,500 kilometres from the farthest point of the country's nine-dashed-line, which Mr. Carpio has called "a gigantic historical fraud."

The government of Vietnam has purchased a copy of the Postal Atlas of China from Mr. Brown. The atlas was printed for several decades in the early 20th century and shows no Chinese authority over islands in the South China Sea.

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Other nations are also "looking to buttress the arguments as to who can claim the sea as their territorial waters," said Barry Ruderman, owner of Barry Lawrence Ruderman Antique Maps in La Jolla, Calif.

Several of his clients "are very specifically focused on defining China or its neighbours through maps," he said. "They're definitely asking for specific groups of islands and specific concepts."

China, meanwhile, has sought to marshal its own claims. Later this year, Chinese researchers will publish two books compiling old maps.

"People who are familiar with Chinese documents and maps will know that China had the earliest records of the Diaoyu Islands" – known as the Senkaku Islands in Japan, which also claims them – "and the islands in the South China Sea," said Chen Jiarong, a senior consultant at the China Maritime Navigation History Research Institute, who has worked on the two books.

Still, international law has placed strict limits on the usefulness of maps, which "merely constitute information, and never constitute territorial titles in themselves alone," the International Court of Justice wrote in a 1986 ruling.

That's doubly true in the China-Philippines case now before the Permanent Court of Arbitration, where "the issues do not include sovereignty over land territory [such as islands]. Thus, even official maps showing claims to islands would not be relevant," said Paul Reichler, the lead counsel for the Philippines side, in an e-mailed response. The court will, instead, rule on the Philippines' maritime entitlements, which directly places into question the validity of China's nine-dashed line.

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China has rejected the arbitration, and refused to participate. It has instead waged war on modern maps it doesn't like.

At the beginning of this year, it outlawed possession of all maps that don't match its regulations, or which could "harm national honour." Authorities have seized globes that do not conform to China's view of the world from foreigners leaving the country. Beijing's sway is obvious on digital mapping software from Apple and Microsoft, which include the dashed line.

But China has struggled to find support even in the most promising of historical documents. Take the Selden map, a 17th-century depiction of trading routes across the South China Sea. It was brought to England by John Selden, a prominent jurist who made the case that countries can "claim jurisdiction over the ocean – the very claim China now makes over the South China Sea," writes Prof. Brook in Mr. Selden's Map of China, a book about the history of the document.

The hand-drawn map, originally bought from a Chinese merchant, is "the only detailed and geographically specific Chinese depiction of these waters before the nineteenth century," he wrote. When it was discovered in an Oxford University library in 2009, it stoked hope that it could "be the winning card in the diplomatic game China plays with its neighbours."

In reality, it does nothing of the sort. "This is a non-political map of East Asia. The only country that's named is Korea," he said.

That's in keeping with the nature of the waters themselves. "China is making unilateral claims over zones that are very much multilateral and international zones," Prof. Brook said.

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The hunt for old maps, meanwhile, has left even the dealers shaking their heads.

"A lot times people come to me and they like a map because it shows this or that island belonging to this or that country. If it's a Western map, the islands are dots. And if the dots happens to be one colour, they buy it. If it happens to be another colour, they don't buy it," said Mr. Brown.

The problem: Those maps were printed in black and white, before being sent to colourists who "weren't scholars," he said. That led to variations in colours used – colours now being interpreted as assigning national ownership – even between different copies of identical maps.

"The notion that a map from 1825 that shows this island or that island belonging to China or to Japan or to Vietnam has any meaning whatsoever is utterly absurd," Mr. Brown said.

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